Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 312202
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
502 PM CDT MON AUG 31 2015

.SHORT TERM...
232 PM CDT

TONIGHT AND TUESDAY...PERSISTENT SURFACE RIDGING ACROSS THE
EASTERN CONUS GENERALLY IS MATCHED BY THE UPPER LEVEL PATTERN
EXCEPT FOR A WEAK CUTOFF SHORTWAVE CIRCULATION ALONG A TROUGH
RUNNING FROM THE OHIO VALLEY INTO THE WESTERN GULF. THIS UPPER
CIRCULATION IS QUITE EVIDENT IN THE WATER VAPOR SATELLITE CHANNEL
BUT IN TERMS OF CONVECTION IT HAS NOT BEEN VERY ACTIVE SO FAR
TODAY ASIDE FROM A BRIEF THUNDERSTORM CLUSTER ACROSS CENTRAL IL.
YESTERDAY SAW MORE WIDESPREAD ACTIVITY STRETCHING FROM OHIO INTO
CENTRAL TEXAS.

MODELS HAVE BEEN DRIFTING THIS TROUGH SLOWLY WESTWARD WHICH MATCHES
THE SATELLITE TRENDS AND LIGHTNING ACTIVITY.  EARLY TOMORROW THIS
TROUGH LIKELY REACHES ITS WESTERNMOST POSITION BEFORE BEING PICKED
UP BY THE NORTHERN STREAM FLOW AND SLOWLY CARRIED OFF TO THE EAST.
THE QUESTION IS WHAT EFFECT IT WILL HAVE ON TSRA CHANCES LOCALLY
THROUGH THE NEXT 24 HOURS.  ACTIVITY TODAY SEEMS TO BE MOST FAVORED
NEAR THE CENTER OF THE TROUGH ALONG THE LOW LEVEL INSTABILITY AXIS
AND WHERE UPPER LEVEL SUBSIDENCE AND THE RELATED CAPPING ARE
WEAKEST. FORECAST SOUNDINGS AND MIDLEVEL TEMPERATURES SUGGEST THE
CUTOFF LINE FOR THE BEST TSRA CHANCES TOMORROW MAY LIE NEAR OR OVER
THE LOCAL FORECAST AREA...WITH OUR SOUTHEASTERN COUNTIES ALONG THE
IL/IN LINE BEING MOST FAVORED.  SOUNDINGS TOWARD THE CHICAGO METRO
AREA AND FARTHER NORTHWEST TOWARD RFD MAINTAIN SLIGHTLY STRONGER
SUBSIDENCE AND CAPPING ALOFT.  FOR NOW WILL CARRY A SLIGHT CHANCE OF
TSRA FOR THE SOUTHEAST PORTION OF THE FORECAST AREA...MAINLY FOR THE
PEAK HEATING PERIOD TOMORROW AFTERNOON.

WITH ALL THIS RIDGING AND MOIST RETURN FLOW OUT OF THE
GULF...TEMPERATURES ARE LIKELY TO APPROACH AND IN A FEW LOCATIONS
EXCEED THE 90 DEGREE MARK TOMORROW.  LOW LEVEL MIXING MAY LIMIT THE
HEAT INDEX VALUES TO THE MID 90S TOMORROW AFTERNOON...BUT REGARDLESS
IT WILL STILL BE A WARM AND HUMID FIRST DAY OF METEOROLOGICAL FALL
ABOUT 10 DEGREES ABOVE NORMAL HIGHS BUT STILL 10 DEGREES BELOW THE
RECORD HIGHS.

LENNING

&&

.LONG TERM...
232 PM CDT

TUESDAY NIGHT THROUGH FRIDAY...
GUIDANCE CONTINUES THE THEME OF UNSEASONABLE WARMTH FOR THE
REGION...AND MOST OF THE AREAS EAST OF THE ROCKIES. 500MB RIDGE
WILL BE STEADILY BUILDING OVERHEAD TUE NGT INTO WED. SOME OF THE
OPERATIONAL SOLUTIONS INDICATE THE LINGERING WEAK MID-LVL WAVE
PROGRESSING EAST THROUGH THE OHIO VALLEY THAT COULD GENERATE AN
ISOLATED SHOWER/THUNDERSTORM TUE NGT/WED...HOWEVER THE FORCING
LOOKS TO REMAIN VERY WEAK. SO HAVE MAINTAINED A DRY FORECAST WITH
HEIGHTS RISING OVERHEAD. THIS SHOULD AID IN KEEPING A LID ON
CONVECTIVE ACTIVITY FOR THE CWFA. THERMAL RIDGE WILL REMAIN OVER
THE FORECAST AREA WED/THUR...WITH HIGHS ABOVE CLIMO AT LEAST IN
THE UPR 80S TO PERHAPS ARND 90. LLVL MOISTURE WILL ALSO BE
PRESENT...WITH DEW POINTS IN THE MID/UPR 60S. THIS WILL CREATE
HUMID CONDITIONS WITH HEAT INDEX VALUES IN THE LOW/MID 90S.

OVERNIGHT LOWS WILL EQUALLY REMAIN MILD...WITH LOWS GENERALLY IN THE
MID/UPR 60S. DEPENDING ON OVERNIGHT CLOUD COVER...A FEW LOCATIONS
MAY REMAIN ARND 70. HAVE NOT INCLUDED A MENTION OF FOG IN THE
EXTENDED PERIODS...HOWEVER WITH THE LACK OF A STRONG GRADIENT AND
LIGHT WINDS OVERNIGHT PATCHY FOG OR HAZE MAY OCCUR.

LATER IN THE WEEK HEIGHTS SHOULD REMAIN OVER THE REGION KEEPING A
LOW CHANCE FOR ANY CONVECTION...AND LIKELY ONLY BEING DIURNALLY
DRIVEN. AFTERNOON LAKE BREEZES WILL BE POSSIBLE AS WELL...WHICH
COULD ALLOW SOME WEAK MESOSCALE FORCING TO HELP TOUCH OFF AN
ISOLATED AFTERNOON STORM OR TWO IN THE EXTENDED PERIODS.

SATURDAY THROUGH MONDAY... HEADING INTO THE HOLIDAY WEEKEND
ENSEMBLES CONTINUE THE THEME OF MID-LVL RIDGING OVER THE EASTERN
CONUS STRETCHING INTO EASTERN CANADA...WITH A TROUGH OVER THE
SOUTHWEST CONUS. SPREAD AMONGST ENSEMBLE MEMBERS IS MINIMAL...WHICH
WILL HELP WITH THE CONFIDENCE OF A CONTINUED UNSEAONABLY WARM PERIOD
INTO THE WEEKEND AND PERHAPS EARLY NEXT WEEK. THUS HAVE HELD ONTO
TEMPS IN THE MID/UPR 80S. THERE CONTINUES TO BE PERIODIC CHANCES FOR
SHOWERS AND THUNDERSTORMS.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* LAKE BREEZE THIS AFTERNOON TO SWITCH WINDS EAST BUT BELOW 8KT.

* MVFR FOG DEVELOPMENT TONIGHT.

MM/BMD

//DISCUSSION...UPDATED 18Z...

CURRENT RADAR DEPICTS LAKE BREEZE MAKING INROADS IN COOK COUNTY
WITH TIME OF ARRIVAL TO ORD/MDW IN THE 20-21Z HOUR. NETWORK OBS
INDICATE WINDS IN THE 090-120 RANGE WITH SPEEDS AT 4-8KT BEHIND
THE LAKE BREEZE. OTHERWISE...HIGH PRESSURE OVER THE TN VALLEY WILL
KEEP HAZY/SCATTERED CLOUD CONDITIONS PRESENTING VFR WEATHER THROUGH
THE DAY.

CONDITIONS REMAIN FAVORABLE FOR FOG TONIGHT...BUT NOT NEARLY AS
DENSE AS LAST NIGHT. A SLIGHTLY TIGHTER PRESSURE GRADIENT WILL
KEEP WINDS MOVING FROM THE SW AT 4KT...JUST ENOUGH TO PREVENT
SETTLING IN THE BOUNDARY LAYER. TONIGHT WILL ALSO LACK A DRY
LAYER IN THE 2-5 KFT RANGE WHICH NORMALLY WOULD MAXIMIZE FOG
POTENTIAL. GIVEN THE INVERSION...STAGNANT AIRMASS...AND LOW LEVEL
MOISTURE...WILL INCLUDE HIGH END MVFR FOG IN THE TAFS FOR TUESDAY
MORNING. DAYTIME TUES WILL BE SIMILAR TO TODAY BUT A 15-20KT
ELEVATED SW WIND WILL LIKELY PIN THE LAKE BREEZE CLOSE TO SHORE.

MM

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* HIGH ON LAKE BREEZE DIRECTION AND SPEED TODAY...MEDIUM HIGH ON
  TIMING.

* MEDIUM HIGH ON FOG REMAINING MVFR TONIGHT.

MM/BMD

//OUTLOOK FOR ORD/MDW FOR 00Z WEDNESDAY-12Z MONDAY...UPDATED 12Z...

TUESDAY NIGHT...VFR.

WEDNESDAY...VFR.

THURSDAY...SLIGHT CHANCE OF TSRA.

FRIDAY...VFR.

SATURDAY...VFR.

SUNDAY...SLIGHT CHANCE OF TSRA.

RODRIGUEZ

&&

.MARINE...
232 PM CDT

WEAK LOW PRESSURE OVER MANITOBA WILL MOVE NORTHEAST
TOWARDS HUDSON BAY AND STRENGTHEN TUESDAY. THIS WILL INCREASE THE
GRADIENT FOR THE NORTHERN PORTIONS OF LAKE MICHIGAN LATE TONIGHT
INTO TUESDAY. OTHERWISE THE SOUTHERN HALF OF THE LAKE WILL CONTINUE
IN A SOUTHWEST TO SOUTH FLOW. THEN LARGE RIDGE OF HIGH PRESSURE WILL
REMAIN NEARLY STATIONARY THROUGH THE REMAINDER OF THE WEEK...WITH
GENERALLY SOUTHERLY WINDS.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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